Contents lists available at ScienceDirect Magnetic Resonance Imaging journal homepage: www.elsevier.com/locate/mri Review article NMR difusometry with guest molecules in nanoporous materials Seungtaik Hwang, Jörg Kärger ⁎ Faculty of Physics and Earth Sciences, Leipzig University, Leipzig, Germany ABSTRACT Application of pulsed feld gradient (PFG) NMR to studying molecular difusion in beds of nanoporous materials has given rise to novel insights and paradigm shifts in our understanding, which are reviewed in the present contribution. This gain in information is, in particular, related to the ability of PFG NMR to discriminate between various mechanisms afecting mass transfer in such systems. Examples include, inter alia, the sensitivity of PFG NMR toward transport enhancement in pore hierarchies as well as toward transport resistances acting, in addition to the difusional resistance of the genuine pore space, either on the crystal surfaces or in their interior. 1. Introduction As an omnipresent and fundamental phenomenon in nature, difu- sion is of central importance for transport of the constituents of matter, namely, atoms and molecules [1–3]. Among the existing, non-invasive measuring techniques for investigating molecular difusion, such as conventional uptake and release measurements [4–6] (including zero length column (ZLC) [7,8] and frequency-response [9–11] techniques as refned variants), permeation studies [12–14], quasi-elastic neutron scattering (QENS) [15,16], interference microscopy [17,18] and in- frared microscopy [19–21], it was in particular the pulsed feld gradient technique of nuclear magnetic resonance (PFG NMR – also referred to as pulsed gradient spin-echo (PGSE) NMR, NMR difusometry and q- space imaging) [22,23] which proved to be an especially powerful technique. It has occupied, correspondingly, a most decisive role in difusion studies, ranging from unconstrained molecular difusion in liquids [24–27] to mass transfer of guest molecules in host porous materials [28–31] and in biological systems [32–34]. In particular, convincing demonstrations of the applicability of the PFG NMR technique to zeolites [30,35–38] have secured its frm foot- hold in studies of molecular self-difusion in the intracrystalline pore system of zeolite crystallites. One of the major breakthroughs in the studies was that PFG NMR initiated a dramatic paradigm shift in the interpretation of the intracrystalline difusion in zeolites by proving that a substantial discrepancy existed between the zeolitic difusivity obtained by PFG NMR and the corresponding difusivity deduced from conventional sorption experiments [39]. This discrepancy is due to the fact that the sorption measurements are based on the observation of molecular uptake by crystals, whose rate is afected by not only the intracrystalline difusivity but also the permeability of the molecules through the crystal surface. If this latter efect is not taken into account and data analysis is performed solely under the implication of difusion limitation, the sorption experiments do indeed give rise to an in- tracrystalline difusivity lower than its genuine value measured by PFG NMR in which the infuence of structural surface resistances is excluded [40]. The present review narrates the history of how PFG NMR, based on its ability to provide clear and direct evidence of quite a number of transport-related quantities, did fnally succeed in accomplishing the paradigm shift in our understanding of mass transfer in zeolites and other nanoporous materials. The options for gaining deeper insights are shown to continue to exist up to the present. They are exemplifed with the challenges provided by difusion in hierarchically organized porous materials and the options of PFG NMR for, once again, providing in- sights into the phenomena determining intrinsic mass transfer in such novel materials which, so far, have remained inaccessible by any other measuring techniques. 2. Diffusion in beds of nanoporous crystallites 2.1. The “various” diffusivities As a most important feature of the application of PFG NMR to beds of nanoporous crystallites, it has to be recognized that the type and the magnitude of difusivities signifcantly depend on molecular displace- ments and, hence, on the size of the crystals under study. In other words, depending on the chosen observation time and measuring temperature, the molecular displacements will vary signifcantly, and the length of the mean difusion path relative to the size of the crystals will decide which type of difusion gives rise to the observed NMR data. In principle, three regimes of difusion can be observable: in- tracrystalline difusion, restricted difusion and long-range difusion. https://doi.org/10.1016/j.mri.2018.08.010 Received 26 July 2018; Received in revised form 20 August 2018; Accepted 23 August 2018 ⁎ Corresponding author at: Faculty of Physics and Earth Sciences, Leipzig University, Linnéstrasse 5, 04103 Leipzig, Germany. E-mail address: kaerger@physik.uni-leipzig.de (J. Kärger). Magnetic Resonance Imaging xxx (xxxx) xxx–xxx 0730-725X/ © 2018 Elsevier Inc. All rights reserved. Please cite this article as: Hwang, S., Magnetic Resonance Imaging, https://doi.org/10.1016/j.mri.2018.08.010